1. Field of the Invention
The present invention relates in general to an internal combustion engine equipped with a secondary air supply system for supplying its exhaust system with air for recombustion of the exhaust gases and more specifically to means for preventing the exhaust emission from being deteriorated in case of trouble or failure of the secondary air supply system.
2. Description of the Prior Art
In a prior art automotive internal combustion engine, the air-fuel ratio is controlled by a feedback system as disclosed in Japanese Patent Provisional Publication No. 60-240840. By the feedback system, the air-fuel ratio is maintained within a narrow range at stoichiometry so that a catalytic converter can operate with a high conversion efficiency.
Under a particular engine operating condition, e.g., an idling condition, the control by the feedback system is suspended and the air-fuel ratio is maintained at a fixed value for causing a rich mixture so that stable combustion of the mixture is obtained. When this is the case, the CO and HC contents in the exhaust gases are increased. To cope with this problem, a secondary air supply system is used for supplying secondary air to an exhaust passage upstream of the catalytic converter and downstream of an oxygen sensor so that the catalytic converter is supplied with a sufficient amount of air necessary for oxidation of CO and HC. However, in case of malfunction trouble, e.g., clogging of the air cleaner or supply pipe, etc. of the secondary air supply system, it will become impossible to supply a sufficient amount of secondary air to the catalytic converter under the above described particular operating condition in which the feedback control of the air-fuel ratio is suspended, thus not only disabling the catalytic converter to be heated rapidly to a desired high temperature but allowing the unburned particles of CO and HC to be emitted into the atmosphere to cause deterioration of the exhaust emission.